Next Generation wireless networks (4G/LTE/LTE-Advanced) are being based on the premise of 100 Mb/s data transmission to the handset. Imagine being able to live stream 1080P HD video to your device anytime, anywhere, even as you are travelling by car (though not while in the driver's seat), bus, etc. Sounds like a great idea and many consumers don't understand why this isn't universally available right now at a reasonable price.
The wireless industry has worked persistently on getting the 3G standards, equipment & networks deployed and functioning, but high bandwidth applications like streaming HD video are generally beyond what those standards, equipment and networks were originally designed to handle.
While deploying faster 4G radios in the existing 3G network may provide some performance improvements, this may only provide a limited benefit. 3G networks are based on fairly large antenna base stations (BTSs). In much of the world, BTS sites have been upgraded with fibre optic network connections to be able to handle the more than 10× bandwidth/handset that 4G networks demand.
Unfortunately, that is not the whole story. If you take a 3G BTS site, it could handle a maximum bandwidth (limited by the combination of the speed and spectrum of the radios and the speed of the backhaul connection to the network) of perhaps 100 Mb/s with a fibre network connection and cover an area of perhaps 15-30 square kilometers.
This means that all the wireless devices within the 15-30 square kilometers can share that 100 Mb/s. In a downtown city core that capacity is exhausted very quickly and everyone gets very little bandwidth. If one of those people is using a device that needs all of that 100 Mb/s bandwidth, then it is not available to anyone else. You can see that the current 3G network architecture will simply not work for high bandwidth applications on any commercial scale. To achieve 4G speeds would generally require between 4-10× the number of 3G BTS sites. Given that each of those sites costs anywhere from $50,000 to $250,000, depending on the distance that the fibre has to be laid to reach it, you can see that this architecture is now non-economic.